1. Field of the Invention
The invention relates to an exhaust gas purification device for an internal combustion engine and a method of controlling the exhaust gas purification device.
2. Description of the Related Art
Conventionally, there is known an exhaust gas purification device applied to an internal combustion engine such as a diesel engine mounted on a vehicle or the like, with an exhaust system provided with a PM filter for collecting particulate matters (PM) mainly composed of soot and a catalytic converter having carried thereon an occlusion/reduction type NOx catalyst for carrying out the purification of exhaust gas as to nitrogen oxides (NOx). In this exhaust gas purification device, with a view to recovering the capacity to purify exhaust gas, control for raising the average of a catalyst bed temperature to a target bed temperature through the supply of unburned fuel components to the catalyst is executed.
For example, in the NOx catalyst, the capacity to occlude NOx decreases due to the occlusion of sulfur components such as sulfur oxides and the like. Thus, in the exhaust gas purification device, with a view to recovering the NOx occlusion capacity of the NOx catalyst that has decreased through the occlusion of sulfur components, the sulfur poisoning recovery control for discharging sulfur components from the NOx catalyst is periodically executed. In this control, the average of the catalyst bed temperature of the NOx catalyst is raised to a target bed temperature of about 600 to 700° C. through the supply of unburned fuel components to the NOx catalyst, and the atmosphere around the catalyst is put into a state during rich combustion (hereinafter referred to as the rich combustion atmosphere) in this high-temperature state. Thus, the discharge and reduction of sulfur components from the NOx catalyst are promoted to recover the NOx occlusion capacity.
However, in the sulfur poisoning recovery control, when the atmosphere around the NOx catalyst is continuously made the rich combustion atmosphere, the catalyst bed temperature may excessively rise as a result of the generation of heat through oxidation of unburned fuel components in the NOx catalyst. Thus, in the sulfur poisoning recovery control, a rich period when the atmosphere around the NOx catalyst is made the rich combustion atmosphere through the supply of unburned fuel components to the NOx catalyst and a lean period when the atmosphere around the NOx catalyst is put into a state during lean combustion by stopping the supply of unburned fuel components to the NOx catalyst are alternately repeated. Owing to this alternate repetition of the rich period and the lean period, the average of the catalyst bed temperature can be held at a high temperature of about 600 to 700° C. without causing an excessive rise in the catalyst bed temperature, and sulfur components can be discharged from the NOx catalyst during the rich period when the atmosphere around the catalyst is made the rich combustion atmosphere.
It is desirable to alternately repeat the rich period and the lean period in the sulfur poisoning recovery control as disclosed in, for example, Japanese Patent Application Publication No. 2005-337039 (JP-A-2005-337039) (paragraphs [0023] to [0040], FIGS. 4 and 5), with the intention of restraining the catalyst bed temperature from rising excessively. In this Japanese Patent Application Publication No. 2005-337039 (JP-A-2005-337039), a first lean period, a rich period, and a second lean period are set as a cycle, and the repetition of the rich period and the lean period in the sulfur poisoning recovery control is realized through the repetition of that cycle.
In the case where the rich period and the lean period are thus repeated, the lean period for restraining the catalyst bed temperature from rising excessively is set as the first lean period and the second lean period respectively before and after the rich period within the cycle when the catalyst bed temperature rises. Accordingly, the first lean period as part of the lean period for restraining the catalyst bed temperature from rising excessively elapses before the rich period when the catalyst bed temperature rises. In this case, the catalyst bed temperature can be more appropriately restrained from rising excessively than in the case where the lean period (the first lean period+the second lean period) for restraining the catalyst bed temperature from rising excessively is set after the rich period and the rich period and the lean period are set as a cycle.
This is because the lean period is unlikely to become insufficient by ensuring the first lean period before the rich period in the case where the cycle ends along the way after the lapse of the rich period. If the lean period (the first lean period+the second lean period) for restraining the catalyst bed temperature from rising excessively is set after the rich period to constitute a cycle, the catalyst bed temperature may rise excessively due to an increase in the insufficiency of the lean period in the case where the cycle ends along the way after the lapse of the rich period.
The repetition of the rich period and the lean period with the intention of raising the average of the catalyst bed temperature to the target bed temperature in Japanese Patent Application Publication No. 2005-337039 (JP-A-2005-337039) will now be described in detail. The repetition of the rich period and the lean period in Japanese Patent Application Publication No. 2005-337039 (JP-A-2005-337039) is executed for the sulfur poisoning recovery control, and realized through processes shown below by [1] to [7].
[1] A rich period is determined on the basis of an engine operation state, and a first lean period is determined on the basis of the rich period. [2] A required flow rate as a flow rate of supplied unburned fuel components needed to make the average of the catalyst bed temperature equal to a target bed temperature through the continuous supply of unburned fuel components to the NOx catalyst is calculated.
[3] A required fuel amount as a total amount of supplied unburned fuel components in supplying the unburned fuel components to the NOx catalyst at the required flow rate from a time point corresponding to the start of the first lean period is calculated. [4] The supply of unburned fuel components to the NOx catalyst is stopped during the first lean period, and unburned fuel components are supplied to the NOx catalyst at a flow rate higher than the required flow rate during the rich period after the lapse of the first lean period.
[5] An actual fuel amount as a total amount of unburned fuel components actually supplied to the NOx catalyst from a time point corresponding to the start of the rich period is calculated. [6] A time to a timing when the actual fuel amount becomes equal to or smaller than the required fuel amount after the lapse of the rich period is set as a second lean period, and the supply of unburned fuel components to the NOx catalyst is stopped during the second lean period.
[7] The required fuel amount and the actual fuel amount are reset to their initial values “0” respectively at a time point corresponding to the end of the second lean period. By repeatedly executing the foregoing processes [1] to [7], the repetition of the rich period and the lean period in the sulfur poisoning recovery control, namely, the repetition of a cycle composed of the first lean period, the rich period, and the second lean period is carried out.
As disclosed in Japanese Patent Application Publication No. 2005-337039 (JP-A-2005-337039), in the case where the first lean period, the rich period, and the second lean period are set as a cycle and this cycle is repeated, the rich period may decrease due to changes in the engine operation state during the rich period, and may end as a result. That is, in the case where the rich period that has decreased is shorter than the rich period that has already elapsed, the rich period ends at a time point corresponding to the decrease thereof. In this case, when the actual fuel amount, which increases during the rich period, has not reached the required fuel amount, which increases from the time point corresponding to the start of the first lean period, the second lean period started after the end of the rich period ends simultaneously with the end of the rich period on the basis of the process [6], and a transition to a subsequent cycle is made.
Accordingly, under the situation described above, the first lean period is ensured in a manner corresponding to the rich period that has not decreased yet, the rich period then ends in a period shorter than a period suited for the first lean period, and a cycle ends as a result. Therefore, in this cycle, the lean period (the first lean period) is too long with respect to the rich period. In a state where the lean period is thus too long with respect to the rich period, the amount of heat generated through oxidation of unburned fuel components in the NOx catalyst in raising the average of the catalyst bed temperature to the target bed temperature is insufficient. When this insufficiency of the amount of heat generation successively occurs in consecutive cycles, the average of the catalyst bed temperature may decrease with respect to the target bed temperature.
This problem arises almost commonly in executing other kinds of control for raising the average of the catalyst bed temperature to the target bed temperature as well as in raising the average of the catalyst bed temperature to the target bed temperature in the sulfur poisoning recovery control. Mentionable as concrete examples of the other kinds of control are filter regeneration control and the like. In filter regeneration control, the average of the catalyst bed temperature of a catalyst provided in an exhaust system is raised to a target bed temperature to eliminate the clogging of a PM filter with particulates.